One method of laser lithotripsy known in the art is that disclosed in PCT Publication No. WO 86/6269 of Furumoto et al. Furumoto discusses the use of a laser lithotripter delivering pulsed laser light at a single wavelength in the 350-550 nanometer (nm) range. Although Furumoto contemplates use of pulse energy levels in the range of 5 -200 millijoules (mJ), in practice the energy range for achieving effective fragmentation is in the order of 20-200 mJ. Furumoto's wavelength of 350 -550 nm lies within a range of significant energy absorption by water and hemoglobin, and thus poses risk of damage to the surrounding tissue during treatment. Further, while in its preferred embodiment operating at a wavelength of 504 nm and pulse energy levels of 30-60 mJ, this modality is effective for fragmentation of many urinary and biliary calculi, however, cystine stones, brushite stones, as well as certain calciumoxalate monohydrate and uric acid stones cannot be treated without the use of undesirably higher pulse energy levels and/or shorter wavelengths.
We have investigated the use of a Q-switched alexandrite laser lithotripter to address tissue damage concerns. In our experiments the laser lithotripter delivered pulsed laser light in the wavelength range of 730-780 nm, where the energy absorption by hemoglobin and water is significantly lower than in the 350-550 nm range. This lithotripter was capable of generating energy in the 0-300 mJ range; in practice, the energy range which is effective for stone fragmentation is in the order of 30-120 mJ. A detailed in vitro study of the Q-switched alexandrite laser lithotripter has disclosed that the energy threshold for achieving the plasma sparking at the surface of the targeted calculi, which is the driving force in the fragmentation process, is undesirably high for many stone compositions. By plasma sparking we mean the initiation of dielectric breakdown of the targeted material, a process which includes the emission of a bright flash of white light and the launching of an audible acoustic wave. Indeed, the pulse energy levels and associated peak power densities required to achieve plasma sparking in certain stone compositions exceeds the transmission capabilities of the fused silica fibers preferably used for clinical laser lithotripsy and generates violent forces which propel the calculi and its larger fragments away from the fiber tip. As a result of these phenomena the range of different compositions of calculi which can be effectively fragmented with the Q-switched alexandrite laser lithotripter is less than that obtainable with the Furumoto apparatus.
Laser apparatus have previously been employed for invasive surgery in the human body. For example, U.S. Pat. No. 4,791,927 to Menger describes a dual-wavelength laser scalpel for both cutting and coagulating tissue employing a laser beam in the wavelength range of 500-800 nm for cauterizing bleeding blood vessels through photocoagulation, and a laser bean in the wavelength range from 250-400 nm for cutting tissue by photoablation.
There is, accordingly, a need for an improved lithotripsy method and apparatus which mainly utilizes the laser light in the long (600-900 nm) wavelength range to effectively treat most urinary and biliary calculi and minimize the total laser pulse energy required to obtain effective stone fragmentation.
Accordingly, it is an object of the present invention to provide a method and related apparatus for improved effectiveness of laser lithotripsy.
Another object of the present invention is to provide an effective laser lithotripter which can fragment most calcified matter while exposing the calcified matter and surrounding tissue to minimum amounts of short wavelength light.